A reference implementation of the GMR-1 cipher has now been released. You can download the sourcecode from http://cgit.osmocom.org/cgit/osmo-gmr/tree/src/l1/a5.c.
Here are the most important facts in a nutshell:
- 4 Linear Feedback Shift registers in Fibonacci configuration.
- 81 bits of state
- Registers 1, 2, and 3 of length 19, 22, and 23 bits control the output
- Register 4 of length 17 bits controls the clocking of registers 1, 2, and 3, and does not contribute to the output
- Clocking control is a simple majority clocking, so that registers are clocked 0 or 1 times
- The output combiner is a quadratic function
So in fact, this cipher looks like a A5/2 clone from GSM.